In connection with future legal specifications with regard to the nitrogen emissions of motor vehicles, a corresponding exhaust gas treatment is required. In order to reduce the NOx emission (NOx removal) of internal combustion engines, especially diesel engines with chronologically predominant lean, i.e. oxygen rich, exhaust gas, the so called selective catalytic reduction (SCR) process can be deployed. In this process a defined amount of a selectively active reduction agent is added to the exhaust gas. This can, for example, be in the form of ammonia, which is metered directly as a gas, or can also be derived from a precursor substance in the form of urea or from a urea-water-solution (HWL).
In the German patent DE 10139142 A1 an emission control system of an internal combustion engine is, for example, described, in which an SCR catalytic converter is deployed to reduce the NO emission which reduces the nitro en oxides contained in the exhaust as to nitrogen with the reagent substance ammonia. The ammonia is derived from a urea-water-solution (HWL) in a hydrolysis catalytic converter disposed upstream in front of the SCR-catalytic converter. The hydrolysis catalytic converter converts the urea contained in the HWL to ammonia and carbon dioxide. In a second step the ammonia reduces the nitrogen oxides to nitrogen, whereby water is produced as a byproduct. The exact mechanism has been adequately described in the trade literature (cf. WEISSWELLER in CIT (72), pages 441-449, 2000). The HWL is supplied in a reagent substance tank.
In the German patent DE 19739848 A1 a procedural approach is described, with which the NO emissions of the internal combustion engine before the catalytic converter can be calculated at least approximately from known operating parameters. The point of origin is an engine characteristic map, which is constructed from the load and rotational speed of the internal combustion engine. Additionally, provision can be made for corrections, for example, as a function of the air number lambda.
From the patent EP 1024254 A2 an exhaust gas treatment system of an internal combustion engine is made known, in which an SCR-catalytic converter is likewise deployed to reduce the NOx emissions. Provision is made again for ammonia to be the reagent substance which is derived from a urea-water-solution (HWL) in the exhaust gas tract. The reagent substance rate is established using the amount of fuel injected, the engine rotational speed as well as using at least one parameter of the exhaust gas, for example, the exhaust gas temperature.
In the patent EP 697062 B1 a procedure and a mechanism are described for the controlled introduction of a reagent substance into an exhaust gas containing nitrogen oxide. Provision is likewise made for a SCR-catalytic converter, which requires ammonia as a reagent substance, which is derived from a reagent substance introduced into the exhaust gas tract upstream from the SCR-catalytic converter. At least one parameter of the exhaust gas relevant to the operation, at least one parameter of a catalytic converter relevant to the operation and if need be a parameter of the internal combustion engine relevant to the operation are acquired to determine the NOx emissions before the catalytic converter of the internal combustion engine. Corresponding to the NOx emissions before the catalytic converter, an intermediate value is determined for a reagent substance rate to be specified. This intermediate value is reduced by a reagent substance rate desorbed by the catalytic converter or increased by a reagent substance rate adsorbed by the catalytic converter.
This characteristic of the SCR-catalytic converter, to be able to at least partially store ammonia, can or must be used depending upon catalytic converter type and metering strategy in order to optimize the NOx conversion rates. Additionally, the ammonia storage capability must be known in order to avoid ammonia breaches, as they can occur during dramatic temperature increases. The background for that is that with increasing temperature the ammonia storage capacity of the catalytic converter sinks. An uncontrolled release of stored ammonia resulting from this leads to offensive smells. To monitor an ammonia breach, ammonia sensors are meanwhile known in context with SCR-catalytic converters. On the basis of zeolite layers, these sensors change their electrical conductivity with the ammonia concentration in the gas surrounding them.
As a rule the ammonia storage capability of a catalytic converter is known when new and can be deposited in a liquid level characteristic curve as a function of temperature. The reduction of the ammonia storage capability with the life of the system (deterioration) is, however, not known.
It is, therefore, the task of the invention, to provide a procedure to monitor the performance capability of a catalytic converter, especially its capability to store reduction agents.
It is additionally the task of the invention, to provide a corresponding device.